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  <title>Document</title>
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  <script>
    // typeof：能判断所有值类型，函数。不可对 null、对象、数组进行精确判断，因为都返回 object 。
    console.log(typeof null); // object
    console.log(typeof []);// object
    console.log(typeof {});// object
    console.log(typeof function(){});// function
    console.log(typeof "");// string
    console.log(typeof 23);// number
    console.log(typeof Symbol());// symbol
    console.log(typeof true);// boolean
    console.log(typeof 9n);// bigint
  </script>
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